TEHRAN (Reuters) -- Iran's police chief has accused the opposition of "inciting sedition" after last month's disputed presidential election and said his force would act firmly to uphold the law.

"This group of individuals voices the slogan of law abidance but they themselves do not observe the law...they are sheer liars," the official IRNA news agency quoted Esmail Ahmadi-Moghaddam as saying on July 21.

He accused the opposition of seeking to sow discord in the Islamic Republic, adding that "security forces should...act firmly in line with the defense of the law.”

Deputy police chief Ahmad-Reza Radan said there were rumors of new "illegal gatherings" in Tehran later on July 21 but that the security forces would firmly confront any such protests, the semi-official Fars News Agency reported.

Ahmadi-Moghaddam did not mention names but his comments were an apparent reference to senior pro-reform figures who say the June 12 vote was rigged in favor of hard-line president Mahmoud Ahmadinejad.

Defeated election candidates Mirhossein Mousavi and Mehdi Karoubi have continued to dispute the official election result, even though Supreme Leader Ayatollah Ali Khamenei has endorsed Ahmadinejad's election victory.

"Some people who failed to realize their election goal go on spreading doubt in different ways and then turn that...into inciting sedition," Ahmadi-Moghaddam said in the northeastern city of Mashhad.

The election stirred the most striking internal unrest in Iran since the 1979 revolution and exposed deep rifts in its ruling elite.

At least 20 people died in post-election violence. Mousavi and the authorities blame each other for the bloodshed. Riot police and religious Basij militia eventually suppressed June's street demonstrations, but Mousavi has remained defiant.

Clashes erupted between police and reformist protesters for the first time in weeks in Tehran on July 17 after former president Akbar Hashemi Rafsanjani declared the Islamic Republic in crisis and said there were doubts about the election result.

That statement was a clear challenge to the authority of Khamenei, Iran's most powerful figure.

Reformist former president Mohammad Khatami on Monday weighed in, calling for a referendum on the legitimacy of the Ahmadinejad's government.

A hard-line newspaper editor seen as close to Khamenei accused Khatami of violating the constitution by suggesting such a referendum and suggested he was doing the bidding of Iran's Western enemies.

"They just brought another phase of the West's scenario into action by presenting such a proposal on holding a referendum aimed at creating another wave of chaos," Editor in Chief Hossein Shariatmadari wrote in his Kayhan newspaper.